Integrated circuit devices generally include many active devices such as transistors, diodes or the like in a semiconductor substrate. Field isolation regions are generally used to isolate the active devices from one another.
It is generally known to form field oxide regions in a face of a semiconductor substrate using local oxidation of silicon (LOCOS) methods. Unfortunately, conventional LOCOS methods may form a bird's beak at the boundary between the field oxide region and the active region of the substrate. The bird's beak may reduce the area of the active region, may be difficult to control, and may cause crystal defects in the substrate due to differences in the thermal expansion coefficients.
In order to reduce or eliminate some of these problems, it has been proposed to perform local oxidation of silicon after first forming a patterned pad insulation layer using a thin film of silicon dioxide or silicon oxynitride. The use of a pad insulation layer may reduce or eliminate the formation of a bird's beak.
Unfortunately, the pad insulation layer may produce a sharp slope at the edge of the field oxide layer relative to the substrate surface of the active region. This sharp slope may generate a steep step which may adversely impact subsequent microelectronic fabrication processes and which may degrade the reliability of the semiconductor device. For example, the steep step may adversely affect a subsequent gate insulation layer and subsequent gate forming processes of field effect transistors.
A conventional isolation method for a semiconductor device and the problems caused thereby will now be described in detail with reference to FIGS. 1A-1D. Referring now to FIG. 1A, a pad insulation layer 15 and an oxidation preventing layer 20 are sequentially formed on the face of a substrate 10. The pad insulation layer 15 is preferably a thin film which prevents bird beak formation during subsequent oxidation, and which may be formed of silicon oxynitride.
Referring now to FIG. 1B, an opening 25 is formed to thereby produce patterned oxidation preventing layer 20a and patterned pad insulation layer 15a. Thus, the patterned oxidation preventing layer 20a and the patterned pad insulation layer 15a remain in the active region of the substrate 10 but are removed from the isolation region thereof.
As shown in FIG. 1C, the substrate 10 is oxidized to form a field oxide region 30 in the face of the semiconductor substrate 10. The field oxide region 30 extends above the substrate face and includes an oblique surface which extends from above the substrate face to the substrate face. As shown, the patterned pad insulation layer 15a reduces or prevents generation of a bird's beak.
Then, as shown in FIG. 1D, the patterned oxidation preventing layer 20a and the patterned pad insulation layer 15a are removed to expose the active region of the substrate 10.
Unfortunately, as illustrated in FIG. 1D, the angle .theta..sub.1 of the field oxide layer 30 with respect to the substrate surface is increased, thereby resulting in a steep step. The steep step may prevent uniform coverage of subsequent layers and may otherwise degrade subsequent processes and structures.